The invention relates to X-ray systems and X-ray lithography utilizing synchrotron radiation, and more particularly to an X-ray lithography beamline imaging system.
The invention also relates to commonly owned U.S. Pat. No. 5,031,199, incorporated herein by reference.
In the manufacture of microelectronic devices, photolithographic techniques are commonly utilized. To obtain greater resolution in the formation of microstructures than can be obtained with visible light wavelengths, efforts have been made to use shorter wavelength radiation, particularly X-rays. To achieve adequate resolution, for example, 0.25 micron lithography, the beam of X-rays must display high spectral and spatial uniformity at the plane of the wafer being exposed. Synchrotrons are particularly promising X-ray sources for lithography because they provide a very stable and defined source of X-rays. The electrons orbiting inside the vacuum enclosure of the synchrotron emit electromagnetic radiation as they are bent by the magnetic fields used to define the path of travel. This electromagnetic radiation is an unavoidable consequence of changing the direction of travel of the electrons and is typically referred to as synchrotron radiation. The energy that the electrons lose in the form of synchrotron radiation must be regained at some point in their orbit around the ring, or they will spiral in from the desired path and be lost. Orbiting electrons can also be lost through collisions with residual gas atoms and ions within the vacuum chamber. Thus, ultra-high quality vacuums are necessary to obtain satisfactory lifetimes of the stored beam.
Synchrotron radiation is emitted in a continuous spectrum of "light", ranging from radio and infrared wavelengths upwards through the spectrum, without the intense, narrow peaks associated with other sources. The shape of a spectral curve of a representative synchrotron storage ring, the Aladdin ring, is shown in FIG. 1. FIGS. 1-4 and a portion of FIG. 5 of incorporated U.S. Pat. No. 5,031,199 are reproduced herein as FIGS. 1-5, respectively, and use like reference numerals where appropriate to facilitate understanding.
All synchrotrons have similar curves as in FIG. 1 that define their spectra, which vary from one another in intensity and the critical photon energy. The critical photon energy E.sub.c is determined by the radius of curvature of the path of the electrons and their kinetic energy and is given by the relationship: ##EQU1## where R.sub.m is the bending radius, m.sub.e is the electron's rest mass, h is Plank's constant, E.sub.e is the energy of the electron beam and c is the speed of light. Half of the total power is radiated above the critical energy and half below. The higher the kinetic energy of the electrons, or the steeper the bend of the orbit, the higher the critical photon energy. By knowing this information, the synchrotron can be designed to match the spectral requirements of the user.
Parameters describing the size of the source of synchrotron radiation and the rate at which it is diverging from the source are also of importance. Since the electrons are the source of synchrotron radiation, the cross section of the electron beam defines the cross section of the source. Within the plane of the orbit the light is emitted in a broad, continuous fan, which is tangent to the path of the electrons, as illustrated in FIG. 2--which shows a section of a synchrotron 20 having an orbiting electron beam 21 and a fan of synchrotron radiation indicated by the arrows 22. FIG. 3 shows the distribution of the flux of the synchrotron radiation at a plane perpendicular to the plane of the ring, with the distribution of flux indicated by the density of the dots shown within the box 25 in FIG. 3. The flux is substantially uniform horizontally, as shown in the graph at 26, and exhibits a Gaussian distribution profile vertically as shown by the graph 27 in FIG. 3.
Because of the relatively small height and width of the electron beam, it acts as a point source of radiation, providing crisp images at an exposure plane which is typically 8 meters or more away from the ring. However, at a distance of 8 meters a 1 inch wide exposure field typically collects only 3.2 milli-radians of the available radiation. There are two ways to improve the power incident at a photo-resist: either shorten the beamline or install focusing elements. The use of focusing elements has the potential advantage of collecting X-rays from a very wide aperture and providing a wide image with a very small vertical height. However, the use of focusing elements results in a loss of power at each element because of low reflectivity of the X-rays and introduces aberrations. To operate within acceptable values of reflectivity and maximize the delivered power, it is necessary to work at grazing angles (i.e., at angles of incidence e from a normal to the surface such that 86.degree. .ltoreq..THETA..ltoreq.89.5.degree. ). Furthermore, because synchrotron radiation is emitted in a horizontal fan, the use of grazing incidence optics is particularly suitable. The small vertical divergence of the synchrotron radiation implies that a wide horizontal mirror can accept a large fan of light at a small grazing angle without being unacceptably long.
The optical system (beamline) must deliver uniform power over the exposure area, typically 2 inches horizontally by 1 inch vertically. This can be achieved by (a) expanding the X-ray beam or (b) scanning the X-ray beam across the image. The first approach is not compatible with vacuum isolation. The present invention is well suited to the second approach, both in the form of mask-wafer scanning and beam rastering.
An X-ray lithography beamline suitable for production purposes should deliver a stable and well characterized flux of X-rays to the exposure field. Desirable characteristics for an X-ray lithography beamline for production purposes include uniform power density over the entire scan region, large collection angle near the source, minimal losses of useful X-rays, a modular optical package with stable, inexpensive recoatable optical elements, and an exposure field measuring at least 1 inch by 1 inch and preferably 2 inches by 2 inches.
Various beamline designs have been proposed for use in X-ray lithography. These include straight-through transmission systems, for example as in B. Lai et al, "University of Wisconsin X-Ray Lithography Beamline: First Results" Nucl. Instrum. Methods A 246, pp. 681 et seq., (1986); H. Oertel et al, "Exposure Instrumentation For the Application of X-Ray Lithography Using Synchrotron Radiation", Rev. Sci. Instrum. 60(7), pp. 2140 et seq., 1989. Other systems have utilized planar optics to provide scanning and filtering capabilities. See, H. Betz, "High Resolution Lithography Using Synchrotron Radiation", Nucl. Instrum. Methods A 246, pp. 659 et seq., 1986; P. Pianetta et al, "X-Ray Lithography and the Stanford Synchrotron Radiation Laboratory", Nucl. Instrum. Methods A 246, pp. 641 et seq., 1986; S. Qian et al, "Lithography Beamline Design and Exposure Uniformity Controlling and Measuring", Rev. Sci. Instrum. 60(7) pp. 2148 et seq., 1989; E. Bernieri et al, "Optimization of a Synchrotron Based X-Ray Lithographic System", Rev. Sci. Instrum. 60(7), pp. 2137 et seq., 1989; U.S. Pat. No. 4,803,713 to K. Fujii entitled "X-Ray Lithography System Using Synchrotron Radiation"; E. Burattini et al, "The Adone Wiggler X-Ray Lithography Beamline", Rev. Sci. Instrum. 60(7), pp. 2133 et seq., 1989. The use of single figured mirrors is proposed in the article by J. Warlaumont, "X-Ray Lithography in Storage Rings", Nucl. Instrum. Methods A 246, pp. 687 et seq., 1986. Other proposed systems include the use of Bragg reflections from crystalline surfaces as described in U.S. Pat. No. 4,028,547 entitled "X-Ray Photolithography" and microfabricated structures as described by R. J. Rosser, "Saddle Toroid Arrays: Novel Grazing Incidences Optics for Synchrotron X-Ray Lithography", Blackett Laboratory, Imperial College, London, England.
In an X-ray lithography system, the X-rays are directed through an X-ray mask and onto the photo-resist in those areas which are not shadowed by the non-transmissive pattern formed on the X-ray mask. Generally, the mask will consist of a thin substrate layer which is overlaid by an X-ray absorbing material in the desired pattern. The transmission of the X-ray mask substrate and the absorption of the photo-resist can be used to define the efficiency of the mask/resist system. Low energy X-rays striking the mask substrate are readily absorbed by the substrate material and never make it to the photo-resist. The energy of these absorbed photons goes into heating the mask, which can lead to undesirable side effects as expansion and distortion of the mask. Very high energy X-rays pass through the mask substrate, the absorber, and the photo-resist with few of the interactions that lead to image formation, reducing the usefulness of these photons. On the other hand, those high energy photons that do interact with the photo-resist may have passed through the absorber or "dark" areas of the pattern on the mask, thus reducing the contrast of the image produced in the resist. The product of the mask transmission and the photoresist absorption defines the system response. Thus, it is preferable that the X-ray flux which reaches the X-ray mask be mainly composed of photons which have an energy which lies in an optimal energy region referred to as the "Process Window". The Process Window will vary depending on the mask substrate and the photo-resist chosen, but in general the Process Window will be in the range of 600 eV to 2000 eV, as illustrated in FIG. 4 for the case of the 2 micron thick polycrystalline silicon mask substrate and a 1 micron thick Novolack photo-resist.
Various beamline designs have been implemented in the prior art, including two and three mirror systems, and a single cylindrical mirror system. The X-ray optics involve simple surfaces such as spherical, cylindrical, elliptical or toroidal. Such surfaces are all symmetrical about an axis. When the requisite imaging does not have a symmetrical property, however, single and symmetric surfaces cannot meet the imaging requirement. Thus, multiple surfaces have been used to correct each others'aberration and deliver required uniformity.
In incorporated U.S. Pat. No. 5,031,199, an X-ray beamline apparatus receives synchrotron radiation X-rays and collects and focuses the beam utilizing two grazing incidence X-ray mirrors which sequentially deflect the beam. The first or entrance mirror is a toroidal mirror which is concave along its length and width. It acts to collect the diverging fan of synchrotron radiation and to collimate partially the X-rays horizontally. The second or refocusing mirror is a concave-convex mirror which is concave in length but convex along its width. The refocusing mirror acts to collimate the light horizontally and to focus the light vertically. The curves of the reflecting surfaces of the two mirrors act in concert to provide a substantially uniform image with uniform power distribution. The two radii of curvatures of the two mirrors, the distance of separation between them, and the inclination angle of the refocusing mirror provide 6 degrees of freedom that can be used to optimize the shape of the image at the exposure field. Parameters of the two mirrors work in concert to produce a better shaped image than either mirror alone. In addition to focusing and collimating the beam, the two mirrors serve to attenuate the high energy photons, e.g., those above approximately 2,200 electron volts (eV). Low energy photons (below 600 eV) are attenuated by a window closing the end of the beamline, preferably formed of beryllium, although a variety of other materials may be used for the window, such as silicon, silicon nitride, silicon carbide and diamond. The beamline system effectively acts as a bandpass filter of photon energies to provide a spectral through-put that closely matches the desired Process Window, resulting in excellent carrier/absorber contrast and good photo-resist response, while simultaneously reducing the heat load on the mask. To obtain scanning of the beam across the image field, a third flat mirror may be interposed in the beamline. This mirror is mounted to pivot slightly about an axis perpendicular to the beam at a low grazing angle of incidence to deflect the beam across the image field as desired. Each of the mirrors is preferably housed within its own self-contained vacuum chamber, with sectioning dual gate valves between these chambers providing the capability of isolating an individual component. Each element of the system can be removed for modification, maintenance or repair without affecting the other elements. Very slight changes in the location and tilt of the two toroidal mirrors can be used to alter the distance to the final image without compromising either the power or the uniformity of the image shape. The resulting beam at the image plane is very sharply defined and substantially uniform in flux across the horizontal width of the beam. Variations in flux across the beam can be compensated, if desired, in various ways, including, but not limited to, profiling the thickness of the exit window to achieve greater attenuation at some areas of the beam than in others, by using variable thickness filters and by using shaped beam apertures.
The present invention provides an X-ray lithography beamline imaging system utilizing a single mirror, and satisfies the imaging requirement with an aspherical reflecting surface. The reflecting surface has symmetry only about a plane, and does not have axial symmetry. The surface function is described by polynomials.